Lower alkyl carbonates may be obtained by the reaction of ethylene and propylene glycol carbonates (glycol carbonates) with alcohols in the presence of catalysts. Although this reaction can take place with high selectivity, the processes used to carry it out have a number of disadvantages. In general, the transesterification reaction is relatively slow under normal pressure, so that it is recommended to apply elevated temperatures, in many cases above the boiling point of the alcohol used, which means carrying out the reaction in pressure vessels (EP 1082, EP 1083). Normally, the reaction only proceeds as far as the equilibrium state of the transesterification reaction. After removal from the pressure vessel, the reaction mixture has to be separated very quickly from the catalyst, for example by flash distillation, to prevent the starting compounds from reforming in a reversal of the formation reaction, for example during removal of the lower boiling alcohol by distillation. During separation from the catalyst by distillation, the glycol carbonate still in equilibrium can decompose into carbon dioxide and polyglycols and is thus lost to the transesterification process with a consequent reduction in yield. All the secondary products mentioned interfere with working up. However, even if this removal of the catalyst is satisfactory, several distillations still have to be carried out. First, separation of high boilers from the low boilers is necessary. However, the purification of ethylene glycol by removal of incompletely reacted ethylene glycol carbonate, which is preferably used for the production of the dialkyl carbonates, is not unconditionally possible because both compounds form an azeotrope. A similar difficulty is encountered when methanol, which is the preferred alcoholic component, has to be separated from the dimethyl carbonate formed. These compounds also form an azeotrope which is difficult to separate by distillation (EP 894). In many cases, higher carbonates are also produced by discontinuous reaction of short-chain carbonates, such as diethyl carbonate, dimethyl carbonate, dibutyl carbonate and/or dipropyl carbonate, with alcohols in a stirred tank reactor. During the reaction, the alcohol formed (methanol, ethanol, propanol, butanol) is distilled off so that the equilibrium position is displaced towards the product side. The product alcohol cannot be distilled off in pure form; a mixture of alcohol and short-chain educt carbonate is always obtained. Where dimethyl carbonate is used, a mixture of azeotropic composition (ca. 30% by weight dimethyl carbonate and 70% by weight methanol) is obtained in the most favorable case. This mixture can only be separated by very elaborate processes, for example azeotropic rectification, or so-called hybrid processes (Chem. Ing. Techn. 68 (1996) No. 4, pages 441-444).
DE 41 29 316 describes a process for the transesterification of glycol carbonates with lower alcohols in which lower alkyl carbonates are obtained. In this process, the formation of an azeotrope is avoided and the reaction takes place under mild conditions.
The problem addressed by the present invention was to provide a simple process for the production of higher dialkyl carbonates. Another problem addressed by the invention was to enable higher dialkyl carbonates with a high percentage content of symmetrical dialkyl carbonates to be produced.